Curator's EnsembleProvider.getConnectionString() is not re-called once the session expiration has been injected

https://issues.apache.org/jira/browse/CURATOR-247 added a faux session expiration event insertion. Once this event is inserted, Curator will no longer call EnsembleProvider.getConnectionString(). This can make some use-cases untenable if the desired connection string might change after this point.